Set the Live Preview URL after launching Chrome.

This is a fix for adobe/brackets#4326

When launching Chrome on the mac, set the Live Preview URL _after_ Chrome is launched. For some reason URLs passed on the command line are treated differently than URLs passed after Chrome has started.

This change does have one side effect: if your Chrome startup settings are "Open the New Tab page", you will get a blank tab _and_ your Live Preview page when starting Chrome. Not ideal, but better than hijacking your browser...